We propose experiments to study the effects of extremely low-frequency (ELF) magnetic field exposure on neurological functions and behavior in the rat. From the data of our preliminary experiments, we hypothesize that acute exposure to ELF magnetic fields activates endogenous opioids, which in turn causes a decrease in cholinergic activity in the brain. These changes in central cholinergic activity lead to a deficit in learning and memory functions. We intend to further investigate and characterize this chain of events through the following experiments: (1) A study to investigate the effects of acute exposure to magnetic fields of different combinations of intensity and duration, and of different frequencies on sodium-dependent high-affinity choline uptake (HACU), an index of cholinergic activity, in different regions of the rat brain. (2) A study to investigate the effects of repeated magnetic field exposure on HACU and muscarinic cholinergic receptors in different regions of the brain. (3) A study to investigate the involvement of opioid receptor subtypes (mu, delta, and kappa) on the effect of magnetic fields on cholinergic systems; and (4) A study to investigate the effect of magnetic field exposure on performance in the radial-arm maze, a behavioral task involving spatial memory functions. Since it is well known that radial-arm maze performance depends on central cholinergic activity, behavioral deficits will be correlated with the changes in cholinergic activity in the brain. It is hoped that the data from these proposed studies will identify the exposure conditions that may trigger deleterious effects, reveal the neural mechanisms affected by ELF magnetic fields, and help in the risk assessment and setting of magnetic field-exposure guidelines in the public and occupational environments.